1. Field of the Invention
The present invention relates to a liquid sealed vibration isolating device for use in a power train mount for a motor vehicle, and more particularly, to the device capable of effectively decreasing noises produced by a cavitation phenomenon and of realizing high damping.
2. Description of the Related Art
In an engine mount for a motor vehicle constructed of a liquid sealed vibration isolating device, the reversal of vibration direction after great force input develops negative pressure within a primary liquid chamber so as to produce bubbles in a hydraulic liquid. Then, a high noise may be produced by a cavitation phenomenon that these bubbles burst. Therefore, there is proposed the device comprising the liquid chamber which is partitioned into the primary liquid chamber and a secondary liquid chamber, and a relief valve which is provided in a partition member having an orifice passage. This relief valve is opened at the time of such great force input that the cavitation phenomenon occurs, to relieve the hydraulic liquid so that the occurrence of the cavitation phenomenon is prevented.
Moreover, there is proposed another device comprising the partition member which has a metallic frame member and an elastic body which is engaged in and supported by the frame member. A central section of the elastic body is formed as a thin-walled circular movable diaphragm section so as to absorb the hydraulic pressure fluctuation of the primary liquid chamber, while an outer circumferential section is formed of a thick wall such that the relief valve is integrally provided with this outer circumferential section.
In this case, the force in the rotational direction (the direction of an arrow A in FIG. 10) is applied to the elastic body by the hydraulic liquid which flows into the orifice passage while swirling, so that the elastic body rotates relative to the frame member. Then, the relief valve may be out of alignment with the relief passage to thereby cause inaccurate opening and closing so that the prevention of the occurrence of the cavitation phenomenon becomes unreliable. Therefore, there is known the art that a thick-walled projection is provided on the outer circumferential section of the elastic body and fixed on the frame member to serve as a detent means (see a patent reference 1).
Further, in this case, the relief valves are provided at regular intervals in the circumferential direction. The length in the circumferential direction (circumference) of each of the relief valves is about ⅙ (a sixth) of the full circumference at a divergent angle of about 60° from the center.    Patent reference 1: Japanese patent laid open publication No. JP 2009-2420 A.